I came across this excellent website today, and had to share it. It is a measure of the “Total Precipitable Water” – the amount of moisture in the atmosphere over a particular location.

Click on the image to view the animation

Polar orbiting satellites measure the microwaves coming from the earth in three specific band widths at 19, 22 and 37GHz. The “brightness” of these three bandwidths varies with the total amount of moisture in the atmosphere, in a straight column, beneath the satellite. The produced images create a map like this, but they are static and the temporal resolution is patchy as the image only updates when a satellite flies directly overhead. The data is therefore combined with the wind fields from global climate models, which project the movement of moisture backwards and forwards to the previous and next image. The result is this stunning animation showing the movement of moisture around the world over 72 hours.

I think this is great. The image here really picks out large weather systems in the subtropics. Barreling into Western Australia you can see a large swath of moisture that will likely bring rain to the east coast tomorrow. While yesterday’s rain can be seen slowly moving offshore. Up in the north atlantic, there’s a really strong system slowly moving east. That’s Tropical Storm Nadine, which is about to hit the Azores. It is predicted to shift to a more west-south westerly direction and head towards the Canary islands over the coming week – likely weakening as it goes.

Additionally you can spot the larger scale features of the planet’s weather. The Inter-tropical convergence zone is obvious as a large band of moisture running across the equator of the Atlantic and eastern Pacific at about 10 degrees North. The position of this will move North and South over the course of the year with the changing seasons. Meanwhile the West Pacific Warm Pool shows up really clearly, the large amounts of warm surface water leads to lots of evaporation, and hence lots of moisture in the air.

The whole thing is a great example of how satellites have revolutionized the field of weather forecasting.